Plant photosynthetic carbon fixation is the important pathway in biological CO₂ fixation, and plays an important role in the process of ecosystems carbon cycle. It is quite meaningful to understand the effect of nitrogen enrichment on the plants photosynthetic carbon fixation. The response of carbon sequestration capacity to nitrogen addition depends on the type and nitrogen saturation of the ecosystems. The nitrogen critical load is higher in grassland and wetland ecosystems, and lower in arid and semi-arid desert ecosystem. The distribution of carbon assimilates in plant organs possibly are altered by the nitrogen addition, which depends mainly on plant physiology, growth rhythm and environmental nutrients condition. Because of the diversity of plant species and ecosystems, the effects of nitrogen enrichment on plant photosynthetic carbon fixation are uncertain. The process models for accurately and quantitatively expressing the effects of nitrogen enrichment on fixation and distribution of photosynthetic carbon in plants are still very limited. We suggest to focus on the biogeochemical models of the effects of nitrogen enrichment on distribution of carbon assimilation, and the coupled model of carbon fixation in plants under both nitrogen and phosphorus enhancement. Further more, by using isotope labeling and molecular biology techniques, we need to determine the effects of nitrogen enrichment on fixation and distribution of photosynthetic carbon from ecosystem viewpoint.